1. Field of the Invention
This invention relates to the use of organometallic compounds for improving the productivity of ionic metallocene olefin polymerization catalysts. These catalysts are the reaction product of metallocenes of a Group IV-B transition metal and ionic activator compounds. These ionic metallocene catalyst systems have enhanced productivity over similar catalysts not using the organometallic compounds. These catalysts are useful for the polymerization of olefins, diolefins, cyclic olefins and acetylenically unsaturated monomers to polyolefins having narrow molecular weight distributions and higher weight average molecular weights than heretofore attainable with a like metallocene which is activated to an active catalyst species by reaction with an aluminum alkyl or alumoxane cocatalyst.
2. Background
Ziegler-Natta type catalysts for the polymerization of olefins are well known. The traditional Ziegler-Natta type soluble systems comprise a metal halide activated to a catalyst species by reaction with a metal alkyl cocatalyst, particularly an aluminum alkyl cocatalyst. The activation of these traditional heterogeneous Ziegler-Natta catalysts generates a variety of different active sites. As a consequence of this non-uniformity of the active sites, the catalysts produce polymer products having a broad molecular weight distribution (MWD). Furthermore, the polymer products exhibit broad composition distribution (CD), poor comonomer incorporation and block sequence distribution.
Recently it has been found that active catalysts are formed when a bis(cyclopentadienyl) compound of the Group IV-B metals, including zirconium and hafnium, is activated by an alumoxane. The metallocene-alumoxane catalysts, whether homogeneous or supported, generally possess high activity and are more versatile than conventional Ziegler-Natta type catalysts in that they may be effectively used to produce a variety of polymer products including, for example, high density linear polyethylene (HDPE), linear low density polyethylene (LLDPE), ethylene-propylene copolymer (EP), non-crystalline polypropylene and crystalline polypropylene. The metallocene-alumoxane catalysts also offer the significant advantage over the traditional Ziegler-Natta catalysts of being able to produce polymers with narrow MWD.
While the metallocene-alumoxane catalysts do offer significant advantages over the traditional Ziegler-Natta catalysts, they nevertheless have limitations in practical commercial applications. These limitations include the relatively high cost of the alumoxane cocatalysts. Alumoxane is also air sensitive and difficult to manipulate. Furthermore, the metallocene-alumoxane catalyst, while producing a narrow MHD polymer product, have a limited capability to produce high molecular weight polymers or polymers having a high comonomer content.
European Patent Application 277,003 and 277,004 (1988), which are hereby incorporated by reference, describe a further advance in metallocene catalysts: new metallocene catalysts which do not require either an alkyl aluminum or an alumoxane as an activator. The Group IV-B metallocene catalysts are prepared as a reaction product of a Group IV-B metal metallocene compound and an ionic activator compound. The ionic activator comprises a cation having a donatable proton or which will irreversibly react with at least one ligand contained in the Group IV-B metal compound and a labile, bulky anion. The bulk of said anion is such that upon reaction of the donatable proton with a substituent of a bis(cyclopentadienyl) Group IV-B metal compound, which reacts with proton forming a group IV-B metal cation, the anion of the activator is sterically hindered from covalently coordinating to the Group IV-B metal cation. Hence, as described in our copending applications, an active catalytic species of a metallocene is formed, namely an ionic pair comprising a metallocene transition metal cation paired with a noncoordinating anion of the activator component.
The new metallocene catalyst systems (hereafter referred to as an "ionic metallocene catalysts") eliminate the need for an expensive alumoxane activator. The ionic metallocene catalysts also offer other advantages over the metallocene-alumoxane catalysts such as permitting the production of polyolefin products of narrow MWD and of significantly higher weight average molecular weight at high rates of catalytic activity while also permitting better incorporation of comonomers and the control of the chain end chemistry of the polymer products.
It is believed that the active catalytic species in the metallocene alumoxane catalysts is an ion pair. It is also believed that this ion pair active species is formed through a Lewis acid-Lewis base reaction of two neutral components (the metallocene and the alumoxane) leading to an equilibrium between a neutral, apparently catalytically inactive adduct, and an ion pair complex which is presumably the active catalyst. As a result of this equilibrium, there is a competition for the anion which must be present to stabilize the active Group IV-B metal cation of the active catalyst species. In the case of the ionic metallocene catalyst described herein, the metallocene and the activator react irreversibly and the equilibrium almost exclusively favors the catalytically active ion pair complex. Hence, the new ionic metallocene catalyst has a very high activity and is able to produce polyolefin products of high molecular weight and narrow molecular weight distribution.
It has been discovered that the activity of the active catalytic ion pair species of our ionic catalyst can be unexpectedly and significantly improved by removing impurities contained in the polymerization diluent or the monomer supply, catalyst is used. The most prominent impurities present in a polymerization diluent and/or a monomer are oxygen and water. Despite the most elaborate control, some, although minute, quantity of such impurities will invariably be present in a polymerization diluent and/or the monomer supply.